1. Technical Field
The present disclosure relates generally to ultrasonic non-contacting inspection systems and more particularly to ultrasonic non-contacting inspection systems requiring only a single element sensor to realize high spatial resolution acoustic imagery in one embodiment, and spatially averaged probing of material properties in another embodiment
2. Description of Related Art
Ultrasonic waves are commonly used to probe a variety of materials (workpieces), particularly for thickness gauging and flaw detection. The sound waves have usually been generated with a contact piezoelectric transducer (PZT). The launched waves propagate through the material, reflecting from interfaces (in thickness gauging applications) or internal features (in flaw detection applications). The scattered sound propagates back to the surface of the workpiece, causing the surface to vibrate at the ultrasound frequency. This vibration has been previously detected with a contact PZT similar to the one used to generate the second.
Optical detection techniques, such as those described in C. B. Scruby and L. E. Drain, Laser Ultrasonics, Techniques and Applications, Adam Hilger, New York (1990), pages 325–350, can be used in place of the piezoelectric transducers to remotely detect the workpiece vibrations. Generally, a laser probe beam is directed onto the workpiece. When the surface vibrates it imparts a phase shift onto the reflected beam. This phase shift is detected with a photodetector after mixing the reflected probe beam with a stable reference beam and measuring the amplitude and frequency or phase of the detector output intensity fluctuations. The reference beam originates from the same laser source as the reflected probe beam, and the output signal from the photodetector or an electronic phase detector corresponds to the surface motion.
One problem with laser detection systems is low sensitivity. Typically, the workpiece surface that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. The aberrated, reflected beam produces a “speckle” field distribution on the optical detector that is used to detect the optical interference between the reflected and reference beams. The phase relationship between the reflected probe beam and the reference beam is maintained only over a single “speckle” diameter. Consequently, the phase relationship can be set optimally only for light within the speckle area; light within other speckles will have a different and generally non-optimal phase relationship with the reference beam. The resulting detector signal can thus be thousands of times weaker, due to multiple speckle capture, than would be the case if the surface were a perfect mirror (in which all light would be in a single speckle).
One prior interferometric technique for detecting ultrasound uses a “self-referencing” interferometer that produces an output proportional to a temporal difference vibration signal, rather than to the displacement, of the moving workpiece surface. Time delay interferometry, described in the Scruby et al. book, pages 123–127, is one such technique. In time delay interferometry the probe beam that is reflected from the workpiece surface is split into two interferometer beams and then recombined at a standard photodetector, with one of the beams time-delayed with respect to the other such as by having it traverse a longer distance. The two beams are collinear when they are recombined at the photodetector, and the light intensity at the photodetector is proportional to the velocity of the workpiece surface. Ideally, the reflected readout beam is interfered with a time-delayed replica of itself and the wavefronts of the two interfering beams are substantially matched. Consequently, a phase shift in one leg of the interferometer is common to all speckles, and all speckles can be detected optimally. Unlike a conventional interferometer, which has a flat frequency response to phase shifts, a time delay interferometer has a bandpass type of response. The time delay interferometer suppresses both the low frequency (below ultrasonic frequencies), as well as certain ultrasonic frequency vibrations.
In U.S. Pat. No. 5,585,921, for “Laser-Ultrasonic Non-Destructive, Non-Contacting Inspection System” by the present applicants and others, a phased-array contactless optical excitation and detection scheme is disclosed in which an array of acoustic waves are generated in the workpiece by a short pulse optical transmitter beam with a beam geometry that is tailored to focus the acoustic waves at an inspection site within the workpiece. The acoustic waves are then detected by reflecting an optical readout beam from a vibrating surface of the workpiece and optically interfering it with a reference beam. The readout beam geometry causes it to detect only those acoustic waves that arrive from the focal inspection site; other acoustic waves are out of phase with each other and cancel. The system employs relatively expensive electronic tracking hardware to compensate for large amplitude, low frequency whole body motions typical in an industrial environment. It uses optical summation for beam formation and thus requires path compensation via photorefractive wavefront compensators, which are relatively slow and introduce occasional signal dropouts.
Another U.S. patent by the present applicants and others, U.S. Pat. No. 5,684,592, for “System and Method for Detecting Ultrasound Using Timedelay Interferometry,” discloses another contactless system in which an optical probe beam is again reflected and phase modulated by a workpiece surface that is vibrated by ultrasound. A timedelay interferometer optically interferes the phase modulated probe beam reflection with a time-delayed replica of itself to produce interference fringes that move in accordance with the workpiece surface displacement temporal differences. The fringes are detected by a non-steady-state photo-electromotive-force, or photo-EMF, detector that generates an output signal when the frequency of the fringe motion exceeds a given threshold. While this system is relatively insensitive to rough workpiece surfaces, suppresses low frequency noise and provides high sensitivity without the need for active stabilization, the delay lines require long, cumbersome optical fibers to form the beam with the attendant input-output coupling losses and power limiting nonlinear effects.
In U.S. Pat. No. 6,075,603, by the present applicants, for “Contactless Acoustic Sensing System With Detector Array Scanning and Self-Calibrating,” another contactless system for imaging an acoustic source within a workpiece is disclosed. In this system, an array of discrete optical detectors are arranged in a pattern. A probe beam is directed onto a vibrating surface in a pattern that corresponds to the detector array. The probe beam is reflected onto the detector array and a reference beam is also directed onto the detector array at an angle to the probe beam to produce fringe patterns on the detectors that correspond to the surface vibration pattern. A readout system utilizes the discrete detector outputs to produce an array output signal indicative of at least a size and two dimensional location for the acoustic source relative to the vibrating surface. While this system allows for electronic beam steering and focusing, it includes arrays, which possess many discrete detector elements and associated circuitry. The contents of U.S. Pat. No. 6,075,603 is hereby incorporated herein by reference.
Todd W. Murray, Hemmo Tuovinen, and Sridhar Krishnaswamy, have written an article entitled “Adaptive Optical Array Receivers for Detection of Surface Acoustic Waves.” See Applied Optics, vol. 39, No. 19, pp 3276–3284 (2000). This article describes a method for collecting a number of discrete laser beams and coherently combining them in a nonlinear photorefractive crystal with a coherent reference so that a single beam (the coherent reference) emerges with all phase information of the input beams imprinted onto this output coherent reference beam. The goal of this device is to pre-process a finite number of input beams so that the ensemble can detect surface waves on an object of interest. The disclosed device enables one to coherently combine a number of probe beams, each of which interrogates a given object at strategically located and predetermined locations on its surface, so that any surface wave can be sampled. The specific goal of the crystal is to provide a single output beam, encoded with the phase information from all of the initial probe beams. In order to subsequently determine the nature of any surface wave, the encoded beam then needs to impinge upon a heterodyne or homodyne optical receiver, equipped with a square law detector and yet another coherent reference beam to reveal the desired phase information. Since most photorefractive crystals are slowly responding, the system can only operate over a limited bandwidth of mechanically induced background noise (whose phase noise is undesirable). Moreover, each probe beam, after reflection from the surface of interest, must each be directed separately onto the crystal. Therefore, that which is needed is a novel image relay system that enables one to employ a single probe beam that illuminates the surface in question with a unique optical pattern (selected to probe a given acoustic mode, be it a surface wave, an internal compression wave, etc.). Furthermore, the optical pattern that strikes the surface should preferably be in the form of a continuous optical pattern, as opposed to a countable number of spots, as in the prior art.
In U.S. Pat. No. 6,008,887, by Klein et al., a single beam laser apparatus for measuring surface velocity at acoustic frequencies and surface displacement at ultrasonic frequencies is disclosed. This apparatus includes a source laser and optics for directing a single laser beam at normal incidence to a surface. A photo EMF detector and optics are provided for directing a surface reflected laser beam at the photo EMF detector in order to provide outputs that are directly proportional to all three orthogonal components of surface velocity or displacement. This patent involves a photo-emf sensor with a crossed electrode configuration, so that motion of an optical pattern can be sensed if the pattern moves arbitrarily in the plane, in both orthogonal directions. This enables one to detect in-plane motion of a non-specular (diffusely scattering) object, in which case, an optical probe beam emerges as a highly speckled beam. This beam, when impinging onto the disclosed photo-emf detector, will result in a dynamic, laterally shifting speckle pattern, in response to a corresponding lateral motion of the object. There is no suggestion in this patent about phased-array detection of a specific acoustic mode of a given object. Indeed, this disclosure teaches that a single laser probe beam interrogates a specific location on the surface of the object.
Therefore, there exists a need in the art for an ultrasonic non-contacting inspection system that incorporates the use of a single element photo-EMF sensor to perform phased array sensing of ultrasound signals to reduce the complexity and cost of prior art systems.